ABSTRACT
On the other hand, the occurrence of liquid evaporation is inevitable unless the atmosphere in the immediate vicinity of the drop is completely saturated with the vapor of the liquid. This fact decreases the reliability of the measured contact angles. An initial advancing contact angle will diminish towards a receding contact angle when the liquid constituting the meniscus starts to evaporate. A more complete understanding of how evaporation influences the contact angle of a drop on polymer surfaces in still air or in controlled atmospheric conditions is very important in the surface characterization processes and is the subject of many recent publications [9-17], The evaporation behavior of small droplets of volatile fluids from solid surfaces depends on whether the initial contact angle is less or more than 90°. In the former case, the contact radius remains constant and the contact angle decreases for much of the evaporation time. Picknett and Bexon [9] reported two modes of evaporation of drops on surfaces: that at constant contact angle and that at constant contact area. They also developed a very successful theoretical analysis of each mode and compared the theoretical predictions with the experimental measurements of methyl acetoacetate drops on a poly(tetrafluoroethylene), PTFE, surface [9], Birdi et al. [10, 11] reported the change in the mass and contact diameter of liquid drops placed on solids with time. They observed that the initial rate of evaporation was dependent on the radius of the liquid-solid interface, rb, by assuming a spherical cap geometry [10], A model based on the diffusion of vapor across the boundary of a drop was considered to explain their data. Shanahan
and Bourges [12] pointed out that the liquid evaporation effect on the contact angle measurements seemed to have been largely neglected. They have shown the existence of three stages in the drop evaporation process in open air conditions. In the first stage, rb remains constant while Ө and the drop height, h, decrease. In the second stage, h and rb diminish concomitantly, thus maintaining Θ more or less constant for smooth surfaces. This stage does not exist on rough surfaces [13]. In the final stage, the drop disappears in an irregular fashion with h, rb, and Θ all tending to zero. They also showed that when the surrounding atmosphere was completely saturated by the vapor of the given liquid, the contact angle remained constant and they proposed a theory to calculate the diffusion coefficient of the liquid vapor in air. They used a drop model in which the vapor concentration varied between saturation on the drop surface to zero over a stagnation layer thickness covering the drop meniscus [13].
